edit the sound used in f curve
Despite the name, f-curves can contain either curve data or sampled data. The sampled data that is used when you bake a sound to an fcurve is not editable.

There are methods available in python to convert between curves and sampled data but blender does not provide a way to do this through the user interface. There is an addon that adds this to the graph editor here.

After installing and enabling the addon you can convert the baked sound to curves which you can then edit as you wish.